The present invention relates to lead-acid cells and batteries, and, more particularly, to the separators used in valve-regulated lead-acid cells and batteries.
Sealed lead-acid cells (often termed xe2x80x9cVRLAxe2x80x9d cells, viz., valve-regulated lead-acid) are widely used in commerce today. As is known, sealed lead-acid cells utilize highly absorbent separators, and the necessary electrolyte is absorbed in the separators and plates. Accordingly, such cells may be used in any attitude without electrolyte spillage as would occur with a flooded electrolyte lead-acid battery. Such cells are normally sealed from the atmosphere by a valve designed to regulate the internal pressure within the cell so as to provide what is termed an effective xe2x80x9coxygen recombination cyclexe2x80x9d (hence, the use of the terms xe2x80x9csealedxe2x80x9d and xe2x80x9cvalve-regulatedxe2x80x9d)
The advantages that are provided by sealed lead-acid cells and batteries in comparison to conventional, flooded lead-acid electrolyte batteries are substantial and varied. Sealed lead-acid technology thus offers substantial benefits by eliminating maintenance (e.g., cell watering), expense (e.g., acid purchases), environmental concerns (e.g., expensive waste treatment systems and air-borne acid mist), and safety (e.g., acid burns).
It is thus not surprising that sealed lead-acid cells and batteries are widely used in commerce today for various applications that have widely differing requirements. In one type of application, generally termed as stationary applications, lead-acid cells and batteries are used, for example, for load leveling, emergency lighting and commercial buildings, as standby power for cable television systems, and in uninterruptible power supplies. The uninterruptible power supply may be used to back up electronic equipment, such as, for example, telecommunication and computer systems, and even as a backup energy source for entire manufacturing plants. When the principal power supply to the electronic equipment or the like has been cut off, such as during a power outage, the sealed cells (typically many electronically connected together) provide a source of reserve power to allow the telecommunication or computer system to remain operational until the principal power supply can be restored. The uninterruptible power supply also will accommodate short, or intermittent, losses in power, so that the function of the electronic equipment will not be impaired during a brief power outage.
In addition, there are many applications where sealed lead-acid cells and batteries are used in what are termed motive power applications. Sealed lead-acid cells and batteries are thus used as the power source for electric vehicles, fork-lift trucks, and the like.
The operation of VRLA cells and batteries is extremely complex and involves a variety of aspects. One important aspect is that VRLA cells must avoid conditions in service in which the temperature within the cell increases uncontrollably and irreversibly. It has thus been hypothesized that excessive water loss resulting in cell dry-out is the driving mechanism for thermal runaway in such cells. This water loss can be caused by hydrogen gassing at the negative electrode or oxygen gassing at the positive electrode through the electrolysis of water, or both.
As the water content and thus the cell saturation is reduced, the oxygen recombination efficiency increases uncontrollably. Since this recombination reaction is highly exothermic, this tends to heat the cell. As the temperature rises, the cell tends to generate gas; and the recombination processes become even more efficient, thereby further increasing the cell temperature. In similar fashion, water loss increases the cell electrical resistance; and such increased cell resistance increases the cell temperature, thereby further increasing water loss. The cell is in thermal runaway.
Despite thermal runaway being an ongoing issue which must be considered in designing VRLA cells and batteries, the impact of a particular separator design on this issue is not well understood. Indeed, the issue of thermal runaway has been dealt with in other ways, as by the selection of the alloys used for the positive grids in such cells and batteries.
Still further, a basic shortcoming of the separators typically used in VRLA cells and batteries is that, at higher battery temperatures (e.g., above about 50xc2x0 C.), a decline in the mechanical and physico-chemical properties of the separators used has been observed. Such decline in properties leads to a decrease in efficiency of the closed oxygen cycle (viz., the oxygen recombination cycle) and to water loss, which shortens the active life of the cell or battery.
Additionally, a shortcoming of VRLA cells and batteries, in general, is that such cells and batteries have somewhat lower capacity, power and energy performance as compared to flooded electrolyte lead-acid cells and batteries. In an attempt to provide enhanced performance, commercial VRLA cells and batteries typically provide some means of compressing the cell or battery elements (viz., the positive and negative plates with the separators interposed therebetween) so as to maintain contact and thereby increase the battery capacity. Such compression, however, can lead to decline in the efficiency of the closed oxygen cycle, due to the reduced number and volume of the gas channels involved, which loss consequently can result in increased water loss.
Essentially from the inception of VRLA cells and batteries, the separators utilized have been highly absorptive glass mat separators. Such separators are usually, but not necessarily, thicker than the separators used in flooded electrolyte lead-acid cells and batteries and have substantially higher absorptivity. Such separators are often termed as xe2x80x9cabsorptive glass mats.xe2x80x9d Such absorptive glass mat separators are well known in this field, and several companies supply such separator materials.
Over the years, those working in this field have provided various proposals for modifying such absorptive glass mat separators in an attempt to overcome the difficulties of such absorptive glass mats. Thus, one proposal has been to include various amounts of polymer fibers into the glass mat, or to provide a polymer fiber layer introduced into the separator in some fashion. Another proposal involves thin paper pulp layers which are coated with layers of absorptive glass mats on both sides of the paper pulp layer. Other proposals have involved providing multiple layer separators (such as layers having different characteristics, e.g., surface area) and plastic separators filled with silica or the like so as to provide acid-gellifying separators.
To attempt to enhance the performance of VRLA cells and batteries, changes in other aspects of the cell and battery design, too numerous to mention, have been proposed. Nevertheless, currently available VRLA cells and batteries typically have lower cycle life and energy performance than is desired. Accordingly, despite the prior efforts in this field, a need exists for separators which can enhance the performance of such VRLA cells and batteries.
Accordingly, it is an object of the present invention to provide separator materials capable of achieving enhanced performance when used in VRLA cells and batteries.
Another and related object of this invention is to provide VRLA cells and batteries capable of achieving enhanced cycle life and energy performance characteristics.
Yet another object of the present invention is to provide facile methods for making such separators.
Other objects and advantages of the present invention can be seen from the following description of the invention.
In general, the present invention is predicated on the discovery that VRLA cells and batteries having enhanced cycle life and electrical performance can be provided by utilizing absorptive glass mat separators modified by treatment with appropriate polymers. Suitable polymers comprise hydrophobic polymers such as polyolefins, polyvinylchlorides, polyacrylonitriles, and polyesters, amphiphilic copolymers, graft copolymers, and hydrophilic nitrogen-containing, water-soluble polymers. Desirably, surface active agents can be included.
It has been found that modification of the absorptive glass mat separators can be achieved by treating such separators with polymeric emulsions or dispersions of the selected polymer. While other types of emulsions can be used, it is desirable from an environmental standpoint to utilize aqueous polymeric emulsions.
It has been found that such modified absorptive glass mat separators impart enhanced mechanical properties and achieve improved electrical performance characteristics in VRLA cells and batteries. Providing such modified absorptive glass mat separators can thus be achieved in a facile fashion.
Moreover, by utilizing multiple separator layers, the individual layer can be treated so as to provide characteristics tailored more specifically to the location of the separator layer in VRLA cells and batteries. More specifically, as will be discussed in greater detail herein, it is desirable, in a preferred aspect of the present invention, to utilized double layer separators in which the relative hydrophilicity of the layers comprising the separator are different so as to enhance the efficiency of the closed oxygen cycle.